The present invention has a particularly advantageous application, although not at all limiting, in ultra-narrow band wireless communication systems. “Ultra-narrow band” (or UNB) means that the instantaneous frequency spectrum of radioelectric signals transmitted by terminals, towards an access network, is of frequency width less than two kilohertz, even less than one kilohertz.
Such UNB wireless communication systems are particularly suitable for M2M type applications (“Machine-to-Machine”) or “Internet of Things” (“IoT”) type.
In such a UNB wireless communication system, the data exchanges are mainly made over an uplink from terminals towards an access network of said system.
The terminals transmit messages which are collected by base stations of the access network, without having to be associated beforehand to one or more base stations of the access network. In other words, the messages transmitted by a terminal are not sent to a specific base station of the access network, and the terminal transmits its messages on its own initiative by assuming that they can be received by at least one base station.
Such arrangements are advantageous in that the terminal does not need to take regular, intensive measurements, in particular from an electrical consumption viewpoint, to determine the most suitable base station for receiving its messages, and does not need to implement an association procedure with this base station before transmitting towards this base station the messages comprising data.
The complexity lies mainly on the access network, of which each base station must be capable of receiving messages which can be transmitted:                at arbitrary times,        over arbitrary frequencies,        by an arbitrary number of terminals being located in a coverage zone of said base station.        
Thus, each base station receives the messages from all the terminals which are located in its coverage zone and, for example, retransmits wholly or partially data included in each message received towards one or more data servers of the access network.
Such as indicated before, such a UNB wireless communication system does not require, for the terminals, to implement an association procedure with a base station of the access network, and the base stations must cope with it.
However, the retransmission of data included in the messages towards a data server can implement a communication protocol requiring that communication resources are reserved before any data exchange. Given that the base stations of the access network do not know, a priori, when they will receive messages from terminals, and do not know from how many terminals they will receive messages, it is not currently possible to ensure that there will always be enough available communication resources to retransmit the data included in all the messages received.
This results in the data of certain messages needing to be temporarily stored by the base station, until they can be retransmitted to the data server, which leads to a delay in the retransmission which can be critical for certain applications. Data from certain messages can furthermore be lost if the quantity of data to be stored by the base station is greater than the storage capacity of said base station.